Shift mechanism of boat propulsion unit

ABSTRACT

A shift mechanism transmits rotation of a drive shaft of an engine to a propeller shaft by engaging a shift clutch with a forward gear or a reverse gear is provided. The shift clutch includes a dog clutch movably coupled to the propeller shaft in an axial direction and a synchro clutch movably coupled to the dog clutch in the axial direction. When the dog clutch is moved to the forward gear side or the reverse gear side to make a shift-in, an end surface of the synchro clutch is adapted to be frictionally engaged with an abutting surface provided on the forward gear or an abutting surface provided on the reverse gear before dog engagement between a pawl of the dog clutch and a pawl of the forward gear or a pawl of the reverse gear. Through this arrangement, it is possible to provide a shift mechanism of a boat propulsion unit provided with a synchronization mechanism that realizes a smooth shift-in.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a shift mechanism of a boat propulsion unit. More specifically, the present invention relates to a shift mechanism of a boat propulsion unit provided with a synchronization mechanism.

2. Description of the Related Art

A shift mechanism that shifts transmission of power from an engine to a propeller into any one of forward, reverse, and neutral positions has been provided in an outboard motor mounted on a boat. FIG. 8 is a view showing a general construction of an outboard motor 100 mounted at a stern of a hull 101. In the outboard motor 100, an engine 102 is disposed with its crankshaft 103 being vertically oriented and a shift mechanism 105 is disposed between a drive shaft 104 connected to a lower end of the crankshaft 103 and a propeller shaft 106 that is a rotating shaft of a propeller 107.

As shown in FIG. 9, the shift mechanism 105 generally includes a drive gear 108 fixed at the lower end of a drive shaft 104, a forward gear 109 and a reverse gear 110 that are disposed on an outer periphery of the propeller shaft 106 and meshed with the drive gear 108 to rotate in an opposite direction with each other, and a dog clutch 111 rotating unitarily with the propeller shaft 106. Shift-change is performed such that a shift sleeve 112 is moved in an axial direction of the propeller shaft 106 and the clutch 111 coupled to the shift sleeve 112 via a cross pin 113 is engaged with the forward gear 109 or the reverse gear 110.

Generally, engagement between the clutch 111 with the forward gear 109 or the reverse gear 110 is made by engaging a pawl formed at an end of the clutch 111 with a pawl formed on the forward gear 109 or the reverse gear 110, i.e., by the so-called dog clutch.

However, when the dog clutch 111 is moved from the neutral position to the forward gear 109 or the reverse gear 110 to shift-in to the forward gear 109 or the reverse gear 110, the forward gear 109 or the reverse gear 110 is rotating while the dog clutch 111 is not rotating. In this case, if engagement between both pawls is not smoothly made, a shift shock will occur. The shift shock may cause vibration and noises to be imparted to the whole outboard motor or the hull.

In JP-A 2004-276726, in order to solve the above problem, there is provided a method in which when the dog clutch is engaged with the forward gear or the reverse gear, the drive shaft is disconnected from the engine so as to not transmit the engine power to the forward gear or the reverse gear. This allows the dog clutch and the forward gear or the reverse gear to rotate synchronously at an early stage, so that the shock of shift-in can be reduced. Further, a method in which a blocking ring is provided between the dog clutch and the forward gear or the reverse gear and the blocking ring is pressed to the forward gear or the reverse gear before engagement between the dog clutch and the forward gear or the reverse gear is provided. In this method, reducing the rotational speed of the forward gear or the reverse gear to which engine output has not been transmitted allows the dog clutch and the forward gear or the reverse gear to engage synchronously with each other at an early stage, thereby absorbing the shock of shift-in.

However, in these methods, it is necessary to control a mechanism (in JP-A 2004-276726, a mechanism that divides a drive shaft into two and controls connection therebetween via an electromagnetic clutch is provided) that interrupts a connection between a drive shaft and an engine at the exact timing of the shift-in, which causes another problem of complicated configuration.

In JP-A 2004-245349, a method of synchronizing rotations of the dog clutch and the forward gear or the reverse gear at an early stage without the use of the mechanism arranged to interrupt a connection between a drive shaft and an engine is provided. FIGS. 10A and 10B show a configuration of a dog clutch 200 depicted in JP-A 2004-245349, wherein FIG. 10A is a perspective view and FIG. 10B is a front view. As shown in FIGS. 10A and 10B, the dog clutch 200 has a plurality of pawls 200 a, 200 b that are of different heights. At a time of shift-in, high pawls 200 a are first engaged with pawls of the forward gear or the reverse gear in order to synchronize rotation of the dog clutch 200 and the forward gear or the reverse gear and then all the pawls 200 a, 200 b including low pawls 200 b are engaged with the forward gear or the reverse gear. This absorbs the shock of shift-in.

However, in the method described in JP-A 2004-245349, there is a possibility that the high pawls 200 a do not smoothly engage with the pawls of the forward gear or the reverse gear in an initial engagement to synchronize rotation of the dog clutch, which may cause the shock in the initial engagement.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a boat propulsion unit shift mechanism with a synchronization mechanism that achieves a smooth shift-in.

A preferred embodiment of the present invention provides a unique configuration of a shift clutch to be engaged with a forward gear or a reverse gear, the shift clutch including a dog clutch movably coupled to a propeller shaft in an axial direction and a synchro clutch movably coupled to the dog clutch in the axial direction.

More specifically, a shift mechanism of a boat propulsion unit according to a preferred embodiment of the present invention is a shift mechanism of a boat propulsion unit that transmits rotation of a drive shaft of an engine to a propeller shaft by engaging a shift clutch with a forward gear or a reverse gear, in which a shift clutch includes a dog clutch movably coupled to a propeller shaft in an axial direction and a synchro clutch movably coupled to the dog clutch in the axial direction and in which when the dog clutch is moved to the forward gear side or the reverse gear side to make a shift-in, an end surface of the synchro clutch frictionally engages with an abutting surface provided on the forward gear or the reverse gear before dog engagement between a pawl of the dog clutch and a pawl of the forward gear or the reverse gear.

This configuration allows the dog clutch synchronously coupled to the synchro clutch to rotate with the forward gear or the reverse gear to make dog engagement with the forward gear or the reverse gear by frictionally engaging the synchro clutch movably coupled to the dog clutch in the axial direction with the forward gear or the reverse gear prior to the engagement between the dog clutch and the forward gear or the reverse gear. This ensures a smooth shift-in and reduces the shock of shift-in.

A sequence of actions of the synchro clutch and the dog clutch at a time of shift-in in a preferred embodiment of the present invention can be regulated by providing an urging device on the shift clutch arranged to provide an urging force against the synchro clutch to the dog clutch.

That is, when the shift clutch performs a shift-in operation, the synchro clutch is moved integrally with the dog clutch by the urging force applied by the urging device until the end surface of the synchro clutch is moved to abut on the abutting surface provided on the forward gear or the reverse gear from the neutral position of the shift clutch. Further, the dog clutch moves independently of the synchro clutch against the urging force since the end surface of the synchro clutch abuts on the abutting surface provided on the forward gear or the reverse gear until the pawl of the dog clutch makes dog engagement with the pawl of the forward gear or the reverse gear.

The urging device described above may preferably include a detent mechanism. In a case that the synchro clutch is coupled to an outer periphery of the dog clutch, the detent mechanism is preferably defined by a groove defined on the outer periphery of the dog clutch, a spring disposed in the groove, and a ball engaged with a groove defined on an inner periphery of the synchro clutch while being urged by the spring. The detent mechanism also performs a function of positioning the shift clutch to the neutral position.

When the above shift clutch is moved to the neutral position to make a shift-out, the synchro clutch moves integrally with the dog clutch with the urging force by the urging device until the other end surface of the synchro clutch is moved to abut on the abutting surface provided on the forward gear or the reverse gear and then the dog clutch moves independently from the synchro clutch against the urging force to return to the neutral position.

Thus, it is possible to have the dog clutch engaged with the shift clutch synchronously rotating with the forward gear or the reverse gear to make dog engagement with the forward gear or the reverse gear by frictionally engaging the shift clutch movably coupled to the dog clutch in the axial direction with the forward gear or the reverse gear prior to the engagement between the dog clutch and the forward gear or the reverse gear. Thus, it is possible to provide a shift mechanism of a boat propulsion unit provided with a synchronization mechanism that realizes a smooth shift-in.

Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the configuration of a shift mechanism according to a preferred embodiment of the present invention.

FIG. 2A is a cross-sectional view taken along the line IIa-IIa in FIG. 1, FIG. 2B is a cross-sectional view taken along the line IIb-IIb in FIG. 1, and FIG. 2C is a cross-sectional view taken along the line IIc-IIc in FIG. 1.

FIG. 3A is a perspective view of a dog clutch according to a preferred embodiment of the present invention, and FIG. 3B is a perspective view of a synchro clutch according to a preferred embodiment of the present invention.

FIG. 4 is a front view of a forward gear or a reverse gear according to a preferred embodiment of the present invention.

FIGS. 5A-5C are cross-sectional views showing a shift-in operation according to a preferred embodiment of the present invention.

FIG. 6 is a graph showing the change of revolutions of the dog clutch according to a preferred embodiment of the present invention at a time of shift-in.

FIGS. 7A-7C are cross-sectional views showing a shift-out operation according to a preferred embodiment of the present invention.

FIG. 8 is a cross-sectional view showing the configuration of a conventional outboard motor.

FIG. 9 is a cross-sectional view showing the configuration of a conventional shift mechanism.

FIG. 10A is a perspective view showing the configuration of a conventional dog clutch and FIG. 10B is a front view of the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description will be made of preferred embodiments of the present invention with reference to the drawings. The present invention is not limited to the following preferred embodiments.

FIG. 1 is a cross-sectional view showing a configuration of a shift mechanism 10 of an outboard motor according to a preferred embodiment of the present invention. The configuration of the outboard motor including the shift mechanism 10 is similar to that of the conventional outboard motor shown in FIG. 8. The present invention is applicable to an inboard motor and an outboard inboard motor (so-called stern drive) as well as to the outboard motor, for example.

As shown in FIG. 1, a shift clutch 11 includes a dog clutch 12 movably coupled to a propeller shaft 21 in an axial direction and a synchro clutch 13 movably coupled to the dog clutch 12 in the axial direction. The shift clutch 11 is engaged with a forward gear 14 or a reverse gear 15 to transmit rotation of a drive shaft (not shown) of an engine to the propeller shaft 21. The forward gear 14 and the reverse gear 15 are normally engaged with a drive gear 30 fixed to the lower end of the drive shaft and rotate in opposite directions relative to each other.

Operation of the shift clutch 11 is such that when the dog clutch 12 is moved to the forward gear 14 side or the reverse gear 15 side to make a shift-in, an end surface 13 a of the synchro clutch 13 is adapted to be frictionally engaged with an abutting surface 14 b provided on the forward gear 14 or an abutting surface 15 b provided on the reverse gear 15 before dog engagement between a pawl 12 a of the dog clutch 12 and a pawl 14 a of the forward gear 14 or a pawl 15 a of the reverse gear 15.

In other words, the synchro clutch 13 starts rotation synchronously with the rotation of the forward gear 14 or the reverse gear 15 by the frictional force and the dog clutch 12 engaged with the synchro clutch 13 thereby starts rotation unitarily with the synchro clutch 13 by frictionally engaging the synchro clutch 13 movably coupled to the dog clutch 12 in the axial direction with the forward gear 14 or the reverse gear 15 prior to the dog clutch 12. When, the dog clutch 12 is further moved to the forward gear 14 side or the reverse gear 15 side to perform a shift-in, the dog clutch 12 can be brought to dog engagement with the forward gear 14 or the reverse gear 15 with its rotation synchronized with the forward gear 14 or the reverse gear 15. This ensures a smooth shift-in and reduces the shock of shift-in.

A sequence of actions of the synchro clutch 13 and the dog clutch 12 at a time of shift-in of a preferred embodiment of the present invention can be regulated by providing the shift clutch 11 with an urging device arranged to apply an urging force against the synchro clutch 13 to the dog clutch 12.

Here, a detent mechanism 16, for example as shown in FIG. 1 and 2A may preferably be used as the urging device, for example. FIG. 2A is a cross-sectional view taken along the IIa-IIa line in FIG. 1. The detent mechanism 16 includes a groove 17 defined on an outer peripheral surface of the dog clutch 12, a spring 20 disposed in the groove 17, and a ball 18 engaged in the groove 19 defined on an inner peripheral surface of the synchro clutch 13 in a state that the ball 18 is urged by this spring 20.

As shown in FIGS. 1 and 2B, keys 22 arranged on the outer peripheral surface of the dog clutch 12 are coupled to a keyway 23 defined on the inner peripheral surface of the synchro clutch 13, thereby achieving key-coupling between the synchro clutch 13 and the dog clutch 12. FIG. 2B is a cross-sectional view taken along the IIb-IIb line in FIG. 1. The key 22 prevents the synchro clutch 13 from moving in the rotating direction with respect to the dog clutch 12. Any type of coupling, for example, spline-coupling, may be used so long as the synchro clutch 13 is movably coupled to the dog clutch 12 in the axial direction.

As shown in FIGS. 1 and 2C, the dog clutch 12 is coupled to a shift sleeve 24 arranged to move the dog clutch 12 in the axial direction via a cross pin 25. FIG. 2C is a cross-sectional view taken along the IIc-IIc line in FIG. 1. Movement of the dog clutch 12 in the axial direction, i.e., shift-change operation of the shift clutch 11, is controlled by the axial movement of the shift sleeve 24 disposed in a hollow portion of the propeller shaft 21.

FIG. 3A is a perspective view showing a configuration of the dog clutch 12 in which the 6 pawls 12 a to make dog engagement with the forward gear 14 or the reverse gear 15 are arranged at regular intervals. The two keys 22 to be coupled to the synchro clutch 13 are defined on the outer peripheral surface of the dog clutch 12, each respectively positioned in the front and the rear in the axial direction. Further, on an inner peripheral surface of the dog clutch 12, spline grooves 26 are defined in the axial direction to be spline-coupled to an outer periphery of the propeller shaft 21.

FIG. 3B is a perspective view showing a configuration of the synchro clutch 13 made up of a cone clutch in which the end surface 13 a is a conical surface. On the inner peripheral surface of the synchro clutch 13, the keyway 23 is arranged in the axial direction to make key-coupling with the dog clutch 12.

FIG. 4 is a front view showing a configuration of the forward gear 14 or the reverse gear 15. Each of gears 14, 15 is made up of a bevel gear having teeth 27 defined on the outer periphery side to be engaged with the drive gear. On the inner periphery side, the abutting surfaces 14 b, 15 b to be frictionally engaged with the end surface 13 a of the synchro clutch 13 and the pawls 14 a, 15 a arranged to ensure dog engagement with the pawl 12 a of the dog clutch 12 are arranged. The abutting surfaces 14 b, 15 b are preferably conical.

Here, the abutting surfaces 14 b, 15 b of the forward gear 14 or the reverse gear 15 can be arranged in a boundary area between the teeth 27 arranged on the outer periphery side of the forward gear 14 or the reverse gear 15 and pawls 14 a, 15 a arranged on the inner periphery side. Therefore, the forward gear 14 or the reverse gear 15 in a preferred embodiment of the present invention can be made generally equal in size to the conventional one.

In contrast, an outer peripheral surface of the synchro clutch 13 may be coupled to an inner peripheral surface of the dog clutch 12. In this case, the abutting surface 14 b (15 b) to be frictionally engaged with the end surface 13 a of the synchro clutch 13 is defined on the inner peripheral side of the pawls 14 a (15 a) arranged to make dog engagement with the pawl 12 a of the dog clutch 12.

Next, a shift-in operation in which the shift clutch 11 of a preferred embodiment of the present invention makes a shift-in to the forward gear 14 side will be described with reference to FIGS. 5A to 5C.

FIG. 5A is a cross-sectional view showing a state in which the shift clutch 11 is in the neutral position. Neither the dog clutch 12 nor the synchro clutch 13 is engaged with the forward gear 14 or the reverse gear 15. The forward gear 14 and the reverse gear 15 are engaged with only the drive gears (not shown) and idle in opposite directions against the propeller shaft 21. Being urged by the spring 20, the ball 18 disposed in the groove 17 defined on the outer peripheral surface of the dog clutch 12 is engaged with the groove 19 defined on the inner peripheral surface of the synchro clutch 13.

Then, as shown in FIG. 5B, the shift sleeve 24 is adapted to move in the direction indicated by the arrow to move the dog clutch 12 coupled to the shift sleeve 24 via the cross pin 25 to the forward gear 14 side. At this time, the synchro clutch 13 moves unitarily with the dog clutch 12, since it is pressed by the ball 18 urged by the spring 20. Accordingly, the forward gear 14 side end surface 13 a of the synchro clutch 13 abuts on the abutting surface 14 b provided on the forward gear 14. In this case, a positional relationship between the dog clutch 12 and the synchro clutch 13 is adjusted in advance in the neutral position so that the distance between the pawl 12 a of the dog clutch 12 and the pawl 14 a of the forward gear 14 is equal to a predetermined value.

If the dog clutch 12 is further moved to the forward gear 14 side, the dog clutch 12 moves independently from the synchro clutch 13 against the urging force by the spring 20 while the end surface 13 a of the synchro clutch 13 abuts on the abutting surface 14 b of the forward gear 14 to be engaged therewith. At this time, the ball 18 that has been engaged with the groove 19 of the synchro clutch 13 becomes disengaged with the groove 19, and fully embedded in the groove 17 of the dog clutch 12. During the movement of the dog clutch 12, rotation of the forward gear 14 is transmitted to the synchro clutch 13 to start rotation by the friction engagement between the end surface 13 a of the synchro clutch 13 and the abutting surface 14 b of the forward gear 14. At the same time, the dog clutch 12 key-coupled to the synchro clutch 13 rotates unitarily with the synchro clutch 13.

As shown in FIG. 5C, the dog clutch 12 comes into dog engagement with the forward gear 14 while rotating synchronously with the forward gear 14. In these actions, the dog clutch 12 can make a smooth shift-in to the forward gear 14.

When the shift clutch 11 makes a shift-in to the reverse gear 15, similar actions to those in the shift-in with the forward gear 14 can be performed.

FIG. 6 is a graph qualitatively indicating the change of revolutions of the dog clutch 12 from the time when the shift clutch 11 is in the neutral position to the time when the shift clutch 11 makes a shift-in to the forward gear 14 or the reverse gear 15.

The synchro clutch 13 starts synchronized rotation by frictional engagement at time t₁ when the end surface 13 a of the synchro clutch 13 has abutted with the abutting surface 14 b of the forward gear 14 with the abutting surface 15 b of the reverse gear 15. At the same time, the dog clutch 12 starts synchronized rotation. Then, the dog clutch 12 increases the speed of rotation, moves independently from the synchro clutch 13, and comes into dog engagement with the forward gear 14 or the reverse gear 15 at time t₂ when revolutions become R₁. Consequently, the dog clutch 12 rotates at the revolutions of R₂, which is the same as the revolutions of the forward gear 14 or the reverse gear 15 and transmits the rotation of the drive shaft to the propeller shaft.

Here, synchronizing revolutions R₁ of the dog clutch 12 at time t₂ depends on the friction force of the frictional engagement between the synchro clutch 13 and the forward gear 14 or the reverse gear 15 and the time (t₂−t₁) that is frictionally engaged. However, synchronizing revolutions R₁ is not necessarily equal to revolutions R₂ of the forward gear 14 or the reverse gear 15. There are cases in that synchronizing revolutions R₁ does not increase sufficiently. For example, in a case that a sufficient area of the abutting surface 14 b or 15 b of the forward gear 14 or the reverse gear 15 cannot be secured due to spatial limitations, or in a case that a sufficient distance cannot be secured between the shift clutch 11 and forward gear 14 or the reverse gear 15 in the neutral position. Even so, however, as long as the synchronizing revolutions R₁ reaches about 20 to about 30 percent of revolutions R₂ of the forward gear 14 or the reverse gear 15, smooth dog engagement may be accomplished.

Next, a shift-out operation in which the shift clutch 11 makes a shift-out from the forward gear 14 side in accordance with a preferred embodiment of the present invention will be described with reference to FIGS. 7A to 7C.

As shown in FIG. 7A, the shift sleeve 24 moves in the direction indicated by the arrow to move the dog clutch 12 to the reverse gear 15 side. At this time, the synchro clutch 13 moves unitarily with the dog clutch 12, since it is pressed by the ball 18 urged by the spring 20. The reverse gear 15 side end surface 13 a of the synchro clutch 13 abuts on the abutting surface 15 b provided on the reverse gear 15. At this time, the dog clutch 12 is displaced to the forward gear 14 side than the neutral position with respect to the synchro clutch 13. Therefore, the reverse gear 15 side pawl 12 a is distant by a determined value from the pawl 14 a of the reverse gear 15.

Next, as shown in FIG. 7B, if the dog clutch 12 is further moved to the reverse gear 15 side, the dog clutch 12 moves independently from the synchro clutch 13 against the urging force by the spring 20 disposed in the groove 17 of the dog clutch 12 and returns to the neutral position shown in FIG. 7B while the reverse gear 15 side end surface 13 a of the synchro clutch 13 abuts on the abutting surface 15 b of the reverse gear 15 to be engaged therewith.

In this situation, a portion of the ball 18 is positioned to be overlapped with the groove 19 provided on an inner peripheral surface of the synchro clutch 13 in the axial direction of the propeller shaft 21. Accordingly, as shown in FIG. 7C, the ball 18 is re-engaged with the groove 19 defined on an inner peripheral surface of the synchro clutch 13 by the urging force of the spring 20 disposed in the groove 17 defined on an outer peripheral surface of the dog clutch 12, thereby repositioning the synchro clutch 13, whose end surface 13 a has been engaged with the abutting surface 15 b of the reverse gear 15, to the neutral position shown in FIG. 7C. That is, the detent mechanism 16 also performs a function of positioning the shift clutch 11 to the neutral position.

When the end surface 13 a of the synchro clutch 13 abuts on the abutting surface 15 b of the reverse gear 15, rotation of the synchro clutch 13 is in the opposite direction to that of the reverse gear 15. However, the urging force by the ball 18 released from engagement between the groove 19 of the synchro clutch 13 is so small that the synchro clutch 13 is movable to a certain degree in the axial direction of the propeller shaft 21 with respect to the dog clutch 12. Therefore, frictional force by the frictional engagement between the synchro clutch 13 and the reverse gear 15 does not increase and a torque may not be rapidly generated.

The operation described above is also applicable to a case in which the forward running boat is decelerated and makes a shift-change to a reverse running. That is, it is possible to let the dog clutch 12, which rotates together with the propeller shaft in the forward direction, rotate in the same direction of the rotation of the reverse gear 15 by frictionally engaging the end surface 13 a of the synchro clutch 13 with the abutting surface 15 b of the reverse gear 15 before dog engagement between the dog clutch 12 and the reverse gear 15. Thus, when the dog clutch 12 makes dog engagement with the reverse gear 15, a rapid torque change is not applied on the reverse gear 15 in a direction opposite to the engine rotation. Accordingly, a sudden load is not applied on the engine and a smooth engagement of the dog clutch 12 with the reverse gear 15 is achieved by the action of the synchronization mechanism of the synchro clutch 13.

Here, in order to make a smooth disengagement of the end surface 13 a of the synchro clutch 13 from the abutting surfaces 14 b, 15 b of the forward gear 14 or the reverse gear 15 at the time of shift-out, the synchro clutch 13 may be made of different material from that of the forward gear 14 or the reverse gear 15. For example, the synchro clutch 13 may be made of copper alloy (high-strength brass, for example), while the forward gear 14 or the reverse gear 15 may be made of alloy steel, for example. Similar effects as described above may be obtained by providing a groove arranged to preserve lubricant on a portion of the end surface 13 a of the synchro clutch 13.

The shift mechanism 10 according to a preferred embodiment of the present invention allows the dog clutch 12 synchronously rotating to make dog engagement (shift-in) with the forward gear 14 or the reverse gear 15 by frictionally engaging the synchro clutch 13 with the forward gear 14 or the reverse gear 15 prior to the engagement between the dog clutch 12 and the forward gear 14 or the reverse gear 15. There are various aspects of the configuration depending on the characteristics of shift-in to the forward gear 14 or the reverse gear 15.

For example, as a general rule, a boat tends to have forward propulsion. Therefore, the shift-in from the neutral position to the reverse gear 15 requires more torque than the shift-in to the forward gear 14 because at a time of shift-in to the reverse gear 15, it is required to once stop a propeller from forcedly rotating for forward running and then to reverse rotation. Considering the above situation, the tilt angle of the reverse gear 15 side end surface 13 a of the synchro clutch 13 is preferably set larger than that of the forward gear 14 side end surface 13 a. This increases the strength of friction engagement of the synchro clutch 13 with the reverse side, thereby making a smooth shift-in to the reverse gear 15.

It is also possible to increase the strength of friction engagement of the synchro clutch 13 with the reverse side by employing different materials for the forward gear 14 side end surface 13 a and the reverse gear 15 side end surface 13 a of the synchro clutch 13.

Further, considering of higher frequencies of the shift-in to the forward gear 14 compared to the shift-in to the reverse gear 15, when the synchro clutch 13 is in the neutral position, the distance between the reverse gear 15 side end surface 13 a of the synchro clutch 13 and the abutting surface 15 b of the reverse gear 15 may preferably be set larger than the distance between the forward gear 14 side end surface 13 a of the synchro clutch 13 and the abutting surface 14 b of the forward gear 14. This lengthens the period of time in which the reverse gear 15 side end surface 13 a of the synchro clutch 13 abuts on the abutting surface 15 b of the reverse gear 15 at a time of shift-out from the forward gear 14 to the neutral position. Accordingly, the synchro clutch 13 abuts on the abutting surface 15 b of the reverse gear 15 at a lower rotational speed. As a result, frictional wear between the end surface 13 a of the synchro clutch 13 and the abutting surface 15 b of the reverse gear 15 can be reduced.

It is also possible to reduce frictional wear between the end surface 13 a of the synchro clutch 13 and the abutting surface 15 b of the reverse gear 15 by decreasing the tilt angle of the reverse gear 15 side end surface 13 a of the synchro clutch 13.

In the foregoing, the present invention is described with reference to non-limiting preferred embodiments. However, the descriptions are not limitations, and various modifications are of course possible. For example, in the above preferred embodiments, a cone clutch is preferably used as the synchro clutch 13. However, the synchro clutch 13 may have an end surface 13 a having no tilt angle.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. A shift mechanism of a boat propulsion unit arranged to transmit rotation of a drive shaft of an engine to a propeller shaft by engaging a shift clutch with a forward gear or a reverse gear, the shift mechanism comprising: a dog clutch movably coupled to the propeller shaft in an axial direction; and a synchro clutch movably coupled to the dog clutch in the axial direction; wherein the dog clutch and the synchro clutch are arranged such that when the dog clutch is moved to the forward gear or the reverse gear to perform a shift-in operation, an end surface of the synchro clutch frictionally engages with an abutting surface provided on the forward gear or the reverse gear before a pawl of the dog clutch makes dog engagement with a pawl of the forward gear or the reverse gear.
 2. The shift mechanism of a boat propulsion unit according to claim 1, wherein the synchro clutch is coupled to an outer periphery of the dog clutch.
 3. The shift mechanism of a boat propulsion unit according to claim 1, wherein the shift clutch includes an urging device arranged to apply an urging force against the synchro clutch to the dog clutch.
 4. The shift mechanism of a boat propulsion unit according to claim 3, wherein when the shift clutch performs the shift-in operation, the synchro clutch is moved integrally together with the dog clutch by the urging force given by the urging device until the end surface of the synchro clutch is moved to abut on the abutting surface provided on the forward gear or the reverse gear from the neutral position of the shift clutch.
 5. The shift mechanism of a boat propulsion unit according to claim 4, wherein when the shift clutch performs the shift-in operation, the dog clutch moves independently from the synchro clutch against the urging force since the end surface of the synchro clutch abuts on the abutting surface provided on the forward gear or the reverse gear until the pawl of the dog clutch makes dog engagement with the pawl of the forward gear or the reverse gear.
 6. The shift mechanism of a boat propulsion unit according to claim 5, wherein during the relative movement between the dog clutch and the synchro clutch the synchro clutch rotates synchronously with the forward gear or the reverse gear with frictional engagement between the end surface of the synchro clutch and the abutting surface provided on the forward gear or the reverse gear, while the dog clutch coupled to the synchro clutch rotates integrally with the synchro clutch.
 7. The shift mechanism of a boat propulsion unit according to claim 3, wherein the urging device includes a detent mechanism.
 8. The shift mechanism of a boat propulsion unit according to claim 7, wherein the detent mechanism includes a groove arranged on the outer periphery of the dog clutch, a spring disposed in the groove, and a ball engaged with a groove on an inner periphery of the synchro clutch while being urged by the spring.
 9. The shift mechanism of a boat propulsion unit according to claim 7, wherein the detent mechanism positions the shift clutch to the neutral position.
 10. The shift mechanism of a boat propulsion unit according to claim 1, wherein the synchro clutch is key-coupled to the dog clutch.
 11. The shift mechanism of a boat propulsion unit according to claim 10, wherein the dog clutch is coupled to a shift sleeve that moves the dog clutch in the axial direction via a cross pin, and keys defined in both sides of the cross pin on the outer periphery of the dog clutch are engaged with a keyway provided on the inner periphery of the synchro clutch.
 12. The shift mechanism of a boat propulsion unit according to claim 1, wherein the synchro clutch is spline-coupled to the dog clutch.
 13. The shift mechanism of a boat propulsion unit according to claim 1, wherein the synchro clutch is defined by a cone clutch whose end surface is a conical surface.
 14. The shift mechanism of a boat propulsion unit according to claim 13, wherein a tilt angle of the conical surface of one end surface of the synchro clutch is different from that of the other end surface of the synchro clutch.
 15. The shift mechanism of a boat propulsion unit according to claim 13, wherein the abutting surface of the forward gear or the reverse gear is a conical surface.
 16. The shift mechanism of a boat propulsion unit according to claim 1, wherein when the synchro clutch is in the neutral position, the distance between the one end surface of the synchro clutch and the abutting surface of the forward gear is different from the distance between the other end surface of the synchro clutch and the abutting surface of the reverse gear.
 17. The shift mechanism of a boat propulsion unit according to claim 1, wherein when the shift clutch is moved to the neutral position to make a shift-out, the synchro clutch moves integrally together with the dog clutch in response to the urging force applied by the urging device until the other end surface of the synchro clutch is moved to abut on the abutting surface provided on the forward gear or the reverse gear and then the dog clutch moves independently from the synchro clutch against the urging force to return to the neutral position.
 18. The shift mechanism of a boat propulsion unit according to claim 1, wherein the synchro clutch is made of different material from that of the forward gear or the reverse gear.
 19. The shift mechanism of a boat propulsion unit according to claim 17, wherein the synchro clutch is made of copper alloy.
 20. The shift mechanism of a boat propulsion unit according to claim 1, wherein a groove arranged to preserve lubricant is defined on a portion of the end surface of the synchro clutch.
 21. A boat propulsion unit comprising a shift mechanism according to claim
 1. 